An atom has an atomic nucleus with a (+) charge at the center and electrons with a negative charge around it.
Electrons are arranged according to certain rules (wave functions), which are called ‘orbitals.’
In an atom, electrons form a ‘shell (layer),’ and each shell comprises several types of orbitals.
Type of orbital
Each shell in an atom has a different type of orbital.
- The innermost shell (n=1) only has an s orbital.
- The second shell (n=2) has an s orbital and a p orbital.
- The third shell (n=3) has s, p, d orbitals.
- From the fourth shell (n>=4) there are s, p, d, f orbitals.
Each orbital has a fixed number of electrons.
- s Orbital can contain 2 electrons.
- p Orbital can contain 6 electrons.
- d Orbital can contain 10 electrons.
- f Orbital can contain 14 electrons.
Each orbital has a slightly different electrical potential energy.
s < p < d < f
The s orbital has the lowest energy so that the electrons can be the most stable.
If both the s orbital and the p orbital of the same shell are empty, the electrons first try to fill the s orbital first.
Rules for electrons to be filled
In a stable atom, electrons are filled from the innermost shell.
In the same shell, electrons are filled in the order of s, p, d, f orbitals.
In other words, as the energy increases in the order of s → p → d → f, the energy level increases.
When the number of shells exceeds 3, the energy boundary begins to become unclear.
For example, a 4s orbital has a lower energy value compared to a 3d orbital. So, before the 3d orbital is filled with electrons, the 4s orbital is filled with electrons first.
Similarly, before the 4d orbital is filled with electrons, it is first filled in the 5s orbital.
Besides, electrons have the property of being in pairs (even) as can as possible.
Also, there are cases where electrons become stable as they fill the upper orbital first. In this case, an exception occurs.
* The electronic layout shown in the above virtual experiment is based on Wikipedia data.